Why spin wave dispersion diagram has twice the number of modes than expectation.

[KULDEEP-KARGETI]

I have two magnetic atoms in the unit cell and they are anti-ferromagnetically aligned. I expect two modes corresponding to these two atoms. But when I look at E vs q curve, I see two more curves symmetrically opposite to the expected curves. Is this due to time reversal symmetry or something else in the code.

i have attached figure for your reference.
In this figure, orange curve is lying on top of another curve(not visible) which seems fine as i have added isotropic exchange only but why exactly opposite curve along the negative side.

Thanks for your answer.